With the recent advances made in digital printing, some would assume that the screen-printing process is on its way out. But technology breakthroughs in UV-curable screen-printing inks are continuing to set new performance standards and are raising the bar for digital processes to compete with screen printing in terms of speed and substrate versatility. Today, UV screen-printing inks are available to support a broad gamut of specialty products and provide product differentiation and decorative appeal above and beyond what is possible with digital four-color-process printing.

Early in the history of UV inks, formulating chemists had a very small selection of raw materials from which to choose, limiting possible ink developments and the applications with which the inks were compatible. But as experimentation expanded the functionality of UV inks during the past decade, raw-material suppliers began to demonstrate greater interest in the ink market, and today, ink makers can choose from virtually thousands of raw materials. This improved, cooperative effort between the raw-material supplier and the ink manufacturer has led to the development of many custom ink innovations specifically for screen printing.

This article will look at the latest generation of UV inks and explore how they've been formulated to overcome traditional performance limitations. It will consider enhancements that have expanded the versatility of UV inks, including improved monomers, oligomers, and photoinitiators, and identify the expanded range of materials to which the inks will adhere. Among the innovations covered will be new specialty UV inks, their performance characteristics, and the applications that can benefit from them. These recent innovations include the following:

In order to understand the latest technical achievements in UV ink technology and the specific performance enhancements UV inks have undergone, it helps to have a handle on the basics of UV ink chemistry. UV formulations are composed primarily of monomers and oligomers. Monomers are reactive diluents with a low molecular weight. They create a homogenous solution and impart the surface characteristics of the ink. Monomers come in three types: monofunctional monomers with slow reactivity, difunctional monomers with medium reactivity, and trifunctional monomers, which offer the fastest cure and the hardest surface. Monomers are 100% solids and do not release volatile organic compounds into the air as solvents do; once monomers are cured, they become a part of the polymer matrix of the ink film.

While monomers impart an ink's surface characteristics, oligomers determine the ending properties of the cured ink film, including its flexibility, weatherability, and chemical resistance. Oligomers have a high molecular weight and form the chemical backbone of a UV ink. They come in several varieties, including epoxyacrylate, polyester, polyurethane, and other types, which differ in the way they link at a molecular level upon curing. The acrylate group is most commonly used in screen-printing inks because of its broad functionality.

Photoinitiators are the key components in starting and completing the UV-curing process. They absorb UV energy from a light source focused at the print surface. This UV energy causes the photoiniators to fragment into reactive materials that, in turn, start a chemical reaction called polymerization. The polymerization process converts the liquid ink into a solid film. Two types of photoinitiators are commonly used in screen-printing inks: free radical and cationic.

UV light does not have sufficient energy to interact with reactive molecules within a coating and generate free radicals. Therefore, a free-radical photoinitiator is added to the formulation. When the photoinitiator molecules are exposed to a specific wavelength of UV energy, they absorb the light and produce free radicals that trigger the cross-linking process. The result is virtually instantaneous polymerization. Free-radical photoinitiators are among the most popular and are the type used in more than 90% of all UV inks for screen printing.

UV inks formulated with cationic photoinitiators generally incorporate different monomers and oligomers than inks with free-radical photoinitiators. But what really sets these inks apart is the way in which the photoinitiators react. Cationic photoinitiators are generally arylsulfonium salts, which form an acid catalyst when exposed to UV light. This acid catalyst triggers polymerization. Unlike free-radical chemistry, cationic reactions will continue after the UV light has been removed (a condition called dark curing). Cationic photoinitiators are present in only about 5-8% of the UV inks used by screen printers.

The physical state of an ink after curing is substantially affected by the lamp configuration. A cured ink film's final properties depend on four major factors:

With the newest UV inks offering greater opacity and the ability to cure with a thick ink film, it has never been more important to understand what UV exposure is and how it affects the ink. There are a number of interactions between UV screen ink and UV lamps, all of which impact the finished ink's performance and determine the optimum configuration of the curing unit.

Two key factors about the curing process that printers often measure, but frequently misunderstand, are the concepts of irradiance (mW/sq cm) and energy (mJ/sq cm). Irradiance is the radiant power arriving at the surface of the ink. Peak irradiance is the highest power value and it occurs at the focal point of the UV light. If a curing unit's reflector does not properly focus light, the irradiance will be less than optimal and the result could be slower curing speeds or undercured ink films.

UV energy is the product of two independent variables: irradiance and time. One Watt of irradiance per 1 second of time equals 1 Joule of energy. Energy is commonly referred to without indication of irradiance. But it is really the proper combination of the two that creates the best curing conditions. Still, when working with highly pigmented, opaque inks or thick-film deposits, higher irradiance has a greater impact than energy on curing.

Ink formulators work with photoinitiator suppliers to develop inks that are compatible with the UV output of medium-pressure mercury-vapor bulbs found in most curing systems for screen printing. Photoinitiators designed for pigmented systems will typically absorb wavelengths between 330-400 nm. For the formulator, there are two different characteristics of the photoinitiator's absorption curve that must be considered:

1. Which wavelengths of light will be absorbed?

2. What is the strength of the absorption?

It's important to understand that all components of a UV ink absorb UV energy, but not at the same wavelength. This concept is depicted in Figure 1, which compares the absorption profiles of unpigmented initiated UV ink and pigmented initiated ink.

Opaque pigmented inks will differ in response to peak UV irradiance and energy, as well as the UV spectra compared to an unpigmented UV varnish. Photoinitiators developed for curing of pigmented coatings typically have higher molar extinction coefficients between the longer wavelengths (300-450 nm) than those made for clear coatings. Longer wavelength light is essential to provide the required through cure in thick UV coatings. Therefore, it is of utmost importance to match the lamp characteristics to the properties of the coating being cured. This approach optimizes curing speeds and overall productivity.

Screen-printing equipment manufacturers also have improved technology, engineering machines that increase production output with decreased set-up times. Because of the ever-present need for speed, ink manufacturers have had to develop inks that cure faster with less UV exposure. Formulating the ink to match the spectral output of the lamp is the key to successful high-speed printing.

The latest applications for UV ink

Recent innovations have allowed ink makers to create UV inks with advanced adhesion, elongation, flexibility, and water-resistance properties. These latest developments in ink technology have had a major impact in the following markets and applications:

P-O-P and display graphics Multipurpose P-O-P inks are available that provide adhesion to a broad range of substrates, including paper, cardstocks, styrene, expanded PVC, and even more difficult polyolefins. Only within the last few years have single-part UV inks become available that will adhere to polyolefins (polyethylene, polypropylene, etc.) and provide performance characteristics such as flexibility and water resistance for banners and similar applications (Figure 2). This new ink type eliminates the need for additives and reduces the ink waste associated with two-part catalyzed inks. It also reduces ink inventory while maintaining a cost-effective price tag.

Container decorating Some of the greatest strides in UV ink technology have been made for container printing (Figure 3). Reliable and easy to use UV inks were drastically needed in the container market, which was previously inundated with additives and multiple ink systems to address the variety of plastics used to manufacture containers. Now HDPE, LDPE, PET, PVC, PP, and other plastic containers can be decorated with single-part, additive-free ink systems. These inks offer increased opacity, along with greatly improved chemical-resistance properties, and they eliminate the need for additives or catalysts.

Hydroscopic plastics Multifunctional monomers are one of the most recent developments in UV ink technology that ink manufacturers are using to overcome the difficulties of printing hydroscopic stocks, such as polyethylene banners and HDPE materials. These substrates absorb moisture, which can create a microscopic barrier of water between the ink and stock that prevents proper ink adhesion. As result, prints on these materials are at risk from abrasion, driving rains, and wind or debris damage. Inks formulated with the new monomers provide vastly improved water resistance on hydroscopic stocks and are fast curing, provide high gloss, remain color fast, and provide prints that are nearly water proof.

Thermoforming UV inks for thermoforming applications also are relatively new. These inks provide ad-vanced adhesion properties and the flexibility to withstand elongation during the forming process. Thermoformable UV inks can be vacuum formed to draw depths of 8 in. or more (Figure 4). These inks also have led to improved formulations that can survive heat bending, routing, and other finishing procedures on various substrates, including styrene, polycarbonate, acrylics, PETG, PVC, and some metals.

Liquid substrates The same high-elongation resins and monomers used in thermoforming inks also have led to new thick-film UV inks, including formulations that are essentially liquid substrates. In other words, these inks can serve as both the graphic image and the substrate (Figure 5). Depending on the specific ink formulation used, the print can be applied to produce a pressure-sensitive display or a static-cling graphic. This substrate-free approach eliminates the need for expensive stock and diecutting costs while it reduces turnaround times on orders.

Thick-film applications The improved understanding of photoinitiator reaction has allowed ink formulators to create numerous specialty UV inks, including products with high pigment loads and the ability to form thick ink films. What once was considered impossible to cure is now a reality. Today, printers can cure UV inks printed through a coarse 60-thread/in. mesh with a stencil emulsion-over-mesh level of 300 microns or more. Among the thick-film ink products available are thixatropic coatings and inks for selective doming applications, which are expected to see a substantial increase in popularity.

Magnetic-receptive products Another ink development that has resulted from new thick-film technology is magnetic-receptive UV ink. Ideal for P-O-P applications, the ink can be printed on a variety of substrates to make them receptive to magnetic materials--the ink does not transform graphics into an actual magnet, but it allows graphics printed with the ink to be held in place by magnetic surfaces (Figure 6). Applications for which the ink is suited include kiosks, shelf strips, headers, and modular displays where graphics need to be easily interchangeable.

Special-effect graphics

Thick-film UV technology has resulted in several new special-effect inks. Glitter UV inks incorporate large glitter particles that create a shimmering effect in the printed graphic. They require a coarse mesh to minimize particle separation, but are well suited for seasonal and specialty graphics and are available in a wide range of colors. Another recent advancement is long-lasting photoluminescent inks that give off light for up to 14 hours. Other UV formulations have been produced with pearlescent, sparkle, and metallic particles in a range of colors for use on a broad assortment of substrates. And new inks with high pigment loads give printers the ability to print on dark surfaces with excellent color coverage and limited bleed through.

Flame-retardant applications Another hot spot in UV screen-ink technology is flame-retardant UV formulations, which have come about in response to consumer concerns about the safety of printed graphics. Ink and substrate manufacturers have been working together to devise solutions. Although UV inks have a high flashpoint, they will begin to decompose and flame once they reach their combustible limit. There are two ways in which this process can be slowed. One is to use retardants in the ink that resist combustion and reduce or eliminate flame. The other option is to formulate the inks as intumescent coatings, which build up layers of char to inhibit oxygen and thus prevent combustion and suppress flames. But it's important to keep in mind that UV flame-retardant inks alone cannot inhibit fire completely; the substrate's ability to intumesce and/ or suppress flame is equally important.

The age of UV

With the current advancements in raw materials for ink production, there seems to be no end to where UV-curable products may take us in the future. In the early days, basic UV inks were sold with a range of additives to enhance functionality. Printers were forced to play chemist in order to obtain adequate ink performance. But today, ink vendors are putting more technology into the ink and developing products based on customer-driven needs. Raw-material suppliers now are expected to develop products based on the needs of ink formulators--unlike in the beginning, when ink vendors were forced to use a limited range of available ink components.

Competition among ink vendors has forced them to look at new and innovative ways to improve ink performance, as well as find new applications that can benefit from UV screen printing. As a whole, the UV screen-printing market has advanced tremendously due to the development of new UV inks, and the trend undoubtedly will continue as raw-material suppliers and ink manufacturers work to evolve their products and set new performance standards.